Your car may have a decal in the door area that gives the wheel and tire sizes for the different ones that were offered from the dealer.  If not, go to one of the online tire retailers (do an internet search for wheel and tire packages).  Usually those sites allow you to enter your car model and it will allow you to select different sized rims and the correct tires for them.

I do not think fuel economy will be affected as long as you stick with the suggested tire size on the selected wheel. 

I only know this because I searched for the exact same thing for my car (17 inch wheels) with the intent of going down to a 15 inch wheel so I could get the same effect 🙂  I've never changed out to a steel rim but I'm still considering it!